Progress toward a practical Nb-Ge conductor

Properties of high-TcNb-Ge films deposited by sputtering and by chemical vapor deposition (CVD) have been investigated. Results of sputtering in the presence of controlled levels of O2, N2, Si, and of reactive sputtering in Ar-GeH4, suggest that the high-TcA15 phase is impurity- or defect-stabilized. In CVD deposits two tetragonal modifications were found: σ and T2, the latter probably stabilized by C12. High critical current densities, Jc(H, T) of fine-grained sputtered films are attributed to flux pinning on A15 grain boundaries. In coarse-grained CVD films high self-field Jcs, 10⁶to 10⁷A cm^-2at T = 4.2 K, are attributed to pinning on dispersed σ-phase. Comparably high Jc´s were also obtained in CVD A15 films doped with impurities. Low field ac losses p (H,T) were correlated with Jcand coating geometries. The feasibility of fabricating multifilamentary composite conductors by CVD was demonstrated experimentally and a fabrication process for long Nb3Ge CVD tapes is being developed.